1. Field of the Invention
The present invention relates, in general, to apparatus and methods for improving the acoustical performance of high-fidelity loudspeaker transducers.
2. Discussion of Related Art
Listeners using loudspeaker systems (“speakers”) in normal circumstances hear both the direct sound radiation from the speaker and a reflected sound field from reflections from the room boundaries and objects. In music reproduction, the reflected sound field is primarily responsible for the desirable sensation of “spaciousness”. Speakers which enhance the reflected sound field in the listening room will impart a greater sense of spaciousness to the music than speakers which do not enhance the reflected sound field. However, if the reflected sound field is too intense, it may cause sound coloration and reduce the localization and clarity. For music, listeners universally prefer the highly spacious reflected sound of rear firing loudspeakers to the more direct sound of front firing loudspeakers. In the monophonic era, some enthusiasts deliberately aimed the speakers away from the audience to create a directionally enriched sound field. (see, e.g., F. Toole, Sound Reproduction, p 126, 2008). Research has also shown that, for speech, most listeners prefer a lower ratio of reflected to direct sound, although the optimum reflected sound level is still above the direct sound level. (see, W. Klippel: Acoustica 70 p 45-54 1990).
Prior art loudspeakers range from products that are highly directional to almost completely omni-directional. Highly directional loudspeakers provide too much direct sound field to the listener and are lacking in important near reflections that have been shown to improve clarity and intelligibility in addition to adding spaciousness. (see Bradley et al., Journal of the Acoustical Society of America, 113 (6), pp 3233-3244, 2003). The first reflections from highly directional loudspeakers are likely to come from surfaces behind the listener which can reduce the clarity, intelligibility and impression of space. Others have noted that omni-directional speakers produce so much reflected sound that they can “deliver a hopelessly confused stereo image when positioned in a typical living room”. (G. L. Augspurger, Paper Number: 8-022 AES Conference: 8th International Conference: The Sound of Audio (May 1990)). Specifically, research has shown that too much front wall reflection can cause sound coloration and reduce the localization (see also, F. Toole, Sound Reproduction p 116, 2008).
Bipolar loudspeakers exhibit acoustic characteristics between the extremities of highly directional and omni-directional loudspeakers. Bipolar loudspeakers have one set or array of transducers or drivers facing forward to provide the direct sound, and a second identical set of transducers facing rearward in phase to enhance the reflected sound field. The reflected sound field consists of reflected sound from the rear transducers and reflected off-axis sound from the front transducers. Bipolar loudspeakers attempt to balance the clarity requirements for speech reproduction with the spatial requirements for music reproduction, and although they can achieve excellent spaciousness, nevertheless improved speech clarity and reduced sound colorization is desirable.
In 1990 Wolfgang Klippel published the results of a series of experiments where he determined that the “feeling of space” ranks with sound quality as the most important two factors in listener preference. Klippel measured this by running tests first with the speakers facing the listener to obtain a “relatively direct” sound field. Then he reversed the same loudspeakers so that they faced away from the listener create a vast reflected sound field. Klippel found that listeners overwhelmingly preferred the front firing speakers for speech, but the rear firing orientation for music. The primary feature of such bipolar speakers, then, is that the polar response of the speaker influences how the speaker interacts with the room, and it is this interaction, and most particularly the first reflections, that impart a sense of spaciousness to the sound produced by the speaker system.
However, test data now indicates that certain of these first reflections may improve clarity and yet others may reduce clarity. Typically, loudspeakers are “voiced” either by ear, by measurements, or a combination of the two methods. The most common, and generally considered the most important, measurement is the on-axis free-field (anechoic) Sound Pressure Level (SPL) vs. frequency response. However, in a bipolar speaker, there are several interactions between the front and back sound field which disturb the SPL measurement that is made by a microphone. Since humans do not listen as a microphone, humans interpret the complex sound field from a bipolar speaker as an improved sense of spaciousness, but also are sensitive to anomalies that produce distortions in the perceived sound.
In addition, currently available high quality electro-acoustic cone diaphragm transducers, such as may be used in the bipolar systems described above, create additional problems, in that they have deficiencies in their high frequency performance, for at high frequencies destructive interference due to the depth of the cone diaphragm causes irregularities in the frequency response. A waveguide may be used to fill the cone cavity and reduce the high frequency irregularities from the destructive interference, but prior art waveguides, which are tapered towards the front of the transducer with a bullet shaped or cone shaped tip, do not prevent destructive interference from the central section of the diaphragm. Thus, there is a need in high performance audio applications for loudspeaker transducers that reduce or eliminate high frequency distortions caused by destructive interference within the transducer.
Various bipolar, dipolar and “omni-directional” speaker systems from a number of manufacturers have attempted to overcome the foregoing perceived distortions by modifying the rear speaker spectral response, by pointing the speakers upwardly, or by using “reflectors” of dubious acoustic effect to “redirect” the sound field, with varying degrees of success. Other approaches have involved the use of wide front baffles to minimize the interaction of the acoustical output from the front and rear speakers, and in the above-referenced commonly-owned U.S. Pat. No. 5,887,068 to Givogue et al, wherein at least one side-mounted speaker was provided in a rectangular enclosure intermediate the front and rear mounted drivers to provide a configuration intended correct undesirable reinforcement and cancellation in acoustical output that occurs over certain frequency ranges.
The Givogue '068 patent teaches the methods to build a bipolar speaker that should produce a measured smooth, flat, on-axis, anechoic SPL curve for the entire speaker. The design trade-offs needed to realize a speaker meeting the objectives of the Givogue patent are not optimal for achieving the overall sound quality goals met sought by the applicants when developing the Bipolar loudspeaker system of the present invention, however. Givogue's side firing driver may be considered spurious to optimum performance of a BP speaker. Specifically, the artifacts in the measured curve that the Givogue patent attempts to improve are too high in frequency for the side firing speaker (subwoofer) to reproduce without introducing distortion of its own. In all practical applications the side firing driver is low-pass-filtered below the midrange (e.g., less than or below 200 Hz) which is where all the spatial effects really begin to work and where the ripples appear in the measured SPL. Second, the Givogue patent teaches use of independent frequency dividing networks for the front and back drivers specifically to flatten the anechoic on-axis frequency response, with no regard to the individual arrays' front and back frequency response and tonal balance. In practice, this leads to speakers with a rear tonal balance which is quite different from the front tonal balance. The rear SPL of a speaker conforming to the Givogue method is typically deficient in the lower midrange which makes the perception that such a speaker sounds harsh and lacks clarity.
There is a need, therefore, for a loudspeaker transducer configuration and method which overcomes the problems with the prior art and provides a loudspeaker system and improved transducer that reduces or eliminates high frequency distortions caused by destructive interference within the transducer, a more enjoyable sound quality for listeners using these loudspeaker systems in rooms and living spaces.